1. Field of the Invention
The present invention relates to devices and methods for selective actuation of wellbore tools. More particularly, the present invention is in the field of control devices and methods for selective firing of a gun assembly.
2. Description of the Related Art
Hydrocarbons, such as oil and gas, are produced from cased wellbores intersecting one or more hydrocarbon reservoirs in a formation. These hydrocarbons flow into the wellbore through perforations in the cased wellbore. Perforations are usually made using a perforating gun loaded with shaped charges. The gun is lowered into the wellbore on electric wireline, slickline, tubing, coiled tubing, or other conveyance device until it is adjacent the hydrocarbon producing formation. Thereafter, a surface signal actuates a firing head associated with the perforating gun, which then detonates the shaped charges. Projectiles or jets formed by the explosion of the shaped charges penetrate the casing to thereby allow formation fluids to flow through the perforations and into a production string.
Tubing conveyed perforating (TCP) is a common method of conveying perforating guns into a wellbore. TCP includes the use of standard threaded tubulars as well as endless tubing also referred to as coiled tubing.
For coiled tubing perforating systems, the perforating guns loaded with explosive shaped charges are conveyed down hole into the well connected to the end of a tubular work string made up of coiled tubing. One advantage of this method of perforating is that long zones of interest (areas of gas or oil) can be perforated with a single trip into the well. The perforating guns are of a certain length each and are threaded together using a tandem sub. With an explosive booster transfer system placed in the tandem sub, the detonation of one gun can be transferred to the next. This detonation can be initiated from either the top of the gun string or the bottom of the gun string.
TCP can be particularly effective for perforating multiple and separate zones of interest in a single trip. In such situations, the TCP guns are arranged to form perforations in selected zones but not perforate the gap areas separating the zones. If the gap distance is short, the gap area is usually incorporated in the gun string by leaving out a certain number of shaped charges or using blanks. However, the detonating cord carries the explosive transfer to the next loaded area of the gun string.
In wells that have long or substantial gaps between zones, an operator must consider the efficiency and cost of perforating the zones. The zones can be perforated separately via multiple trips into the well, which requires running the work string in and out of the well for each zone to be perforated. This increases rig and personnel time and can be costly.
Referring now to FIG. 1, there is shown another conventional system for perforating multiple zones that includes perforating guns 12 that are connected to each other by tubular work strings 14. Devices such as circulation subs 16 can be used to equalize pressure in the work strings 14. The guns 12 are fired using a detonator body 18 that is actuated by a pressure activated firing head 20. During operation, the operator increases the pressure of the wellbore fluid in the well by energizing devices such as surface pumps. The firing heads 20, which are exposed to the wellbore fluids, sense wellbore fluid pressure, i.e., the pressure of the fluid in the annulus formed by the gun and the wellbore wall. Once a pre-set value of the annulus fluid pressure is reached, the firing heads 20 initiate a firing sequence for its associated gun 12. The firing heads 20 usually incorporate a pyrotechnic time delay 21 to allow operators to exceed the activation pressure of each firing head 20 in the TCP string 10 to ensure each firing head 20 is activated. If the operator cannot increase the pressure in the well, or if one of the firing heads or time delays fails and a zone is not perforated another round trip in the well is required to perforate the zone that was missed on the initial run. Each trip in the well costs time and money.
These conventional firing systems for various reasons, such as capacity, reliability, cost, and complexity, have proven inadequate for certain applications. The present invention addresses these and other drawbacks of the prior art.